1. Technical Field
The present invention relates to methods and devices for the treatment of a gas flows that include guiding the gas flow through a gas treatment unit adapted for filtering particles from the gas flow and eliminating those particles at the gas treatment unit. More particularly, the invention is intended to be used for separating and eliminating particles from the exhaust gases of a combustion engine. The invention also relates to a device for accomplishing such a treatment of a gas flow.
2. Background of the Invention
In the field of vehicles that are operated by combustion engines, there is a general demand for low emissions of harmful substances in the exhaust gases from the engine. The substances are primarily constituted by pollutants in the form of oxides of nitrogen (NOx), hydrocarbon compounds (HC) and carbon monoxide (CO). As regards petrol engines, the exhaust gases are normally purified by means of an exhaust catalyst that forms part of the exhaust system and through which the exhaust gases are guided. In what is referred to as a three-way catalyst, which is previously known, the major part of the above-mentioned harmful compounds is eliminated by means of known catalytic reactions. In order to optimize the function of the catalyst so that it provides an optimal degree of purification for NOx, HC, and CO, the engine is in most operating conditions operated by a stoichiometric air/fuel mixture; that is, a mixture where (lambda)=1.
Furthermore, in the field of vehicles that are operated by means of a combustion engine, there is a demand for low emissions of harmful substances in the form of solid particles, and primarily in the form of carbon particles that are present in the engine exhaust gases. This demand or requirement is, for example, valid in connection with diesel engines. In order to eliminate such particles, it is previously known to utilize various types of carbon filters. For example, today ceramic filters are utilized that are constructed with a porous wall structure by means of which particles in the exhaust gases can be deposited in the pores of the wall structure.
One disadvantage of such ceramic filters is that they must be regenerated at regular intervals; that is, particles that have been accumulated in the filter must be removed from the wall structure after some time of use. In this case, the regeneration suitably takes place by means of combustion of the particles at a certain increased temperature, normally 400-500 degrees C., which for example can be obtained by means of a special electric heater. After this measure, the filter can once again be utilized for accumulating particles.
One particular type of particle filter is the so-called “city filter,” by means of which the necessary temperature can be obtained so that the combustion of particles can take place. Although this type of filter functions satisfactorily in principle, it has certain disadvantages, such as, because of its construction, it imposes a comparatively high fall in pressure thereacross. Furthermore, it has a comparatively low filtration capacity because it is based on surface filtration across a comparatively small surface. Furthermore, the pressure loss over such a filter increases over the years, which is due to the fact that ashes gradually obstruct the filter. In addition, carbon can be accumulated during operation at low temperatures. When the vehicle subsequently increases the load, spontaneous ignition may occur and which can permanently damage the filter.
There are also other types of filters that are based on the fact that particles are gradually accumulated in a filter unit, which in that case is replaced after some time of use.